This is a perfect example of a problem asking for the LCM (Least Common Factor). If every 6th person in line gets a prize, and every 8th person gets a prize, then those are the two numbers you have to use to find the LCM. Now let's list the multiples of both numbers:
6: 6, 12, 18, 24, 30, 36, 42, 48, 54, 60
8: 8, 16, 24, 32, 40, 48, 56, 64, 72, 80
What is the first number you see that both numbers (6,8) have? I see 24 as the first number. So, in the line, the first person to get both the theater ticket and the gift card of $30 will be the 24th one. Hope this helps! :D